1. Field of the Invention
The present invention relates to vapor-liquid contact grids and, more particularly, to corrugated contact plates disposed in face-to-face contact for use in vapor-liquid process towers.
2. History of the Prior Art
In the vapor-liquid contact art, it is highly desirable to utilize methods and apparatus that efficiently improve the quality as well as the quantity of the mass heat transfer occurring in process towers. The technology of such process towers is replete with various material designs used for tower packing. Types of packing are functions of the particular process to be effected within the tower. The packing elements may thus comprise a structured grid array (grid packing) arranged to form a regular array inside the column or may comprise oblique shapes dumped into and randomly arranged (dump packing) within the tower. Close fractionation and/or separation of the feed stock constituents introduced into the tower and the elimination of harmful or undesirable residual elements imparts criticality to the particular vapor-liquid contact apparatus designed. The shape of the dump or grid packing elements determines the flow patterns in and density of the array and the resultant resistance to flow caused thereby. Prior art grid arrays have thus found utility in a variety of shapes, sizes and material forms in both grid and dump packings configurations.
It has been found particularly desirable in the prior art to provide apparatus and methods affording efficient heat transfer, fluid vaporization, or vapor condensing duty whereby cooling of one of the fluids can be accomplished with a minimum pressure drop through a given zone of minimum dimensions. High efficiency, low pressure drop and reduced temperatures are most often found as design criteria in the chemical engineering art as applied to petroleum refraction operations. Process towers for effecting such chemical reactions are generally of the character providing descending fluid flow from an upper portion of the tower and ascending vapor flow from a lower portion of the tower. Sufficient surficial area for vapor-liquid contact is necessary for the primary function and the reduction or elimination of liquid entrainment present in the ascending vapor. Most often it is necessary for the grid array to have sufficient mass and surficial area in both its horizontal and vertical planes so that fractions of the heavy constituents are conducted downwardly in condensed form and the vapors are permitted to rise through the grid with minimum impedence. With such apparatus, undesirable solids or heavy constituents of the feed stock are removed by the coaction of the ascending liquid vapor to provide a self-cleaning grid. A plurality of stacked layers affording compatible and complemental design configurations are generally assembled within a single process column. Each layer utilizes the velocity and kinetic energy of the ascending vapors to perform the dual function of eliminating liquid entrainment in the ascending vapor and the thorough and turbulent contacting of the vapor with the descending liquid to accomplish sufficient separation or fractionation of the fluids into desired components. Quick cooling of the ascending vapor is generally a prerequisite for efficient operation to effect efficient heat transfer for vapor condensation and minimum pressure drop in a minimum vertical depth of the grid. Oppositely inclined corrugated lamella, or plates, have thus been utilized in the prior art for affording multiple vapor passages through the horizontal and vertical planes of the grid layers to insure the flow of vapor and distribution thereof within the lamella and prevent maldistribution, or channeling, of the vapor through certain portion of the layers and not others. Only in this manner is efficient and effective utilization of the column and the energies applied therein effected.
Most often used in the process column prior art is a plurality of layers with grid members of each layer having angularly disposed adjacent elements. Each element generally has a structural configuration and angularity that permits a large upright vapor passage area which is in excess of one-half the horizontal area of the lamella. This design affords acceptable efficiency and vapor-liquid distribution for heat-mass transfer. Such structures also be necessity provide thorough and turbulent mixing or contacting of ascending vapor and descending liquid. This is done without materially displacing either the vapor or liquid from its vertical location within the flow grid. It is important to prevent maldistribution or channeling of either the vapor or the liquid through certain portions of the grid or its layers.
The structural configuration of inclined corrugated contact plates of the prior art variety often incorporate lineal vapor orifice passages. Vapor turbulence is created by such orifices to insure intimate vapor liquid contact. It is also necessary to insure the ascending vapor performs a dual function of liquid contact and liquid disentrainment within close proximity to the vertical location at which the ascending vapor approaches or leaves the vapor passage orifices. In this manner maldistribution of the ascending vapor or descending vapor is prevented. It is, moreover, a tantamount concern of the prior art to provide such methods and apparatus for vapor-liquid contact in a configuration of economical manufacture. Such considerations are necessary for cost effective operation.
Oppositely inclined corrugated plates provide but one method and apparatus for countercurrent, liquid-vapor interaction. With such grid arrays, the liquid introduced at or near the top of the column and withdrawn the bottom is effectively engaged by vapor being introduced at or near the bottom of the column and withdrawn at the top. The critical feature in such methods and apparatus is to insure that the liquid and vapor achieve the desired degree of contact with each other so that the planned reaction occurs at the designed rate within controlled parameters of mass transfer. The internal structure is, of course, passive in the sense that it is not power driven and has few, if any, moving parts. The prior art is thus replete with passive vapor-liquid contact devices utilizing cross-fluted and perforated sheets of material in face-to-face engagement. This configuration encourages the liquid moving through the grid to form itself into films having, in the aggregate, a large area over which the vapor may pass. However, the design problem is not merely a matter of providing a large surface area or a multitude of corrugations, cross-flutes, or perforations. A number of other interrelated design considerations must be taken into account, some of which have been mentioned above.
From a process standpoint, it is important that the desired vapor-liquid contact reaction be carried as close to completion as possible. For example, in a crude oil vacuum tower, close fractionation and good separation are needed to produce gas oil streams that are free of undesirable residual elements. As mentioned above, the contact column and its internal apparatus must efficiently utilize the heat supplied to the unit. In this manner, it minimizes direct operating costs, whether the reaction is mass transfer, heat transfer, liquid-vaporization or vapor condensing duty. With the above, pressure drop is the primary consideration as is the vapor-liquid fluid interface. Such grids for vapor-liquid contact have been shown in the prior art in such references as U.S. Pat. No. 3,343,821, issued Sept. 26, 1967; U.S. Pat. No. 4,139,584, issued Feb. 13, 1979; U.S. Pat. No. 4,128,684, issued Dec. 5, 1978; U.S. Pat. No. 3,785,620, issued Jan. 15, 1974; and U.S. Pat. No. 3,959,419, issued May 25, 1976.
In the above-referenced vapor-liquid contact method and apparatus patents, a plurality of design configurations are presented for affording intimate vapor-liquid contact. In particular, stacked corrugated contact plates in face-to-face contact having corrugations inclined to the horizontal, and/or orthogonal one to the other, have been shown and provided in various material configurations. These configurations include monofilament yarns, and solid plates. It is moreover prominent in the prior art to utilize cross-fluted plates having a myriad of perforations therethrough.
While many prior art methods and apparatus for vapor liquid contact have been shown to be effective, certain disadvantages still remain. In particular, vapor-liquid contact towers incorporating descending liquid flow and ascending vapor flow of the passing grid variety defined above, is generally incapable of self-regulation of internal pressure differentials and the prevention of maldistribution, or non-homogenous, vapor-liquid flow across the grid areas. This is true even with a plurality of aperatures disposed between corrugated and/or cross-fluted plates in face-to-fact contact. Vapor flow is ultimately sensitive to pressure differentials, including laminar flow patterns, and is easily diverted between the myriad of exposed areas of mating corrugations or flutes. Even when the corrugations of adjacent plates are inclined at relatively sharp angles, vapor flowing along any one corrugation is substantially exposed to the adjacent corrugated channels rather than to the film of liquid along wall sections. Vapor-liquid flow in such configurations is thus susceptible to an inefficient, random flow pattern which cannot be accurately determined within the passive grid.
The absence of any substantial degree of flow directionality through surface wall areas within the corrugations decreases the uniformity and homogeniety of the flow pattern throughout the grid along with the programmed turbulence and mass-heat transfer characteristics typical of predefined grid structures with known flow characteristics. Moreover, the notional plane between face-to-face corrugations and/or fluted plates comprises in the main, the largest available planar surface area of any single corrugation section. The surface area is subjected only to vapor flow. This is true because all sheeting of fluid flow must, by definition, be limited to the walled surface area. For this reason, the notional plane of separation between facing corrugations of most prior art structures comprises an area of lost vapor-liquid contact and thus a loss of efficiency in the operation.
It would be an advantage, therefore to overcome the problems of the prior art by incorporating the advantages of face-to-face corrugated and fluted contact plates with utilization of the notional plane of separation therebetween for maximizing efficiency and vapor-liquid contact without adversely effecting the the operational characteristics for adding to pressure losses therethrough. The methods and apparatus of the present invention provide such an improvement over the prior art grid arrays by providing an apertured surface, or plate, disposed along the notional plane between said corrugated plates. In this manner, liquid is permitted to flow both within the corrugations or flutes of facing plates and along the notional separation plane therebetween for substantially increasing the vapor-liquid contact of ascending vapor and descending liquid normally passing between said corrugated planes. The presence of apertures within the sheet also permit flow diversion and the formation of liquid deposits which collect in certain apertures to be exposed on opposite sides thereof directly to vapor flow in opposed directions thereacross. Such liquid vapor flow configurations are in effect maximum utilization of process tower technology and may be afforded with minimal increase in production costs over that of conventional opposed plate corrugation assemblies not incorporating a notional plane separation sheet. Moreover, a variation in the aperture size may further enhance vapor pressure equalization and flow distribution between opposed channels where non-homogeneous flow areas have been created.